The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable to be mounted in a relatively small camera such as a camera built in a portable device such as a cellular phone and a portable information terminal, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In these years, in place of cellular phones that are intended mainly for making phone calls, so-called “smartphones” have been more widely used, i.e., a cellular phones with such functions as those of portable information terminals (PDA) and/or personal computers. Since the smartphones generally are highly functional as opposed to the cellular phones, it is possible to use images taken by a camera thereof in various applications. For example, in case of the smartphone, while it is possible to use for printing and enjoying images taken as of its intended use, it is also possible to use in additional uses such as processing images to be used in games or for makeup simulations, dress fitting simulations, and the others. Such uses of the images, which were not conventionally common, are becoming increasingly popular every year.
Generally speaking, a product group of cellular phones and smartphones is often composed of products with various specifications such as those for beginner users and those for advanced user. Among them, an imaging lens to be mounted in the cellular phone or the smartphone, which is developed for advanced users, is required to have a high resolution lens configuration so as to be also applicable to a high pixel count imaging element of these days. However, as the imaging lens to be mounted in smartpones used for the above-described usages, it is critical to be a small size with a wide angle of view, that is, a wide angle, than having a high resolution. Especially in these days, with advancements in downsizing and high performances of smartphones, an imaging lens has been required to have smaller size and wider angle of view.
Accordingly, in case of cellular phones or smartphones, depending on significance in the product group, there is slight difference in specifications of imaging lens for mounting in cellular phones and smartphones. It is preferred to choose a most preferable lens configuration for each required specification, but in view of solving problems including shortening of the product development period and cost reduction, a five-lens or six-lens configuration is desired. In a lens configuration composed of six lenses, since the number of lenses that compose an imaging lens is many, although it is slightly disadvantageous for downsizing of the imaging lens, there is flexibility in designing, so that there is potential to attain satisfactory correction of aberrations and downsizing of the imaging lens in a balanced manner. As a lens configuration composed of six lenses, for example, the one described in Patent Reference has been known.    Patent Reference Japanese Patent Publication No. 2011-145315
The imaging lens described in Patent Reference includes a first lens that is negative and has a shape of a meniscus lens directing a convex surface thereof to an object side; a bonded lens composed of two lenses, positive and negative lenses; a fourth lens that is positive; and a bonded lens composed of two lenses, positive and negative lenses. According to the imaging lens of Patent Reference, satisfying a conditional expression about curvature radii of an object-side surface and an image plane-side surface of the first lens and conditional expressions of the two bonded lenses, it is achievable to satisfactorily correct a distortion and a chromatic aberration.
According to the imaging lens described in Patent Reference, however, since a distance from the object-side surface of the first lens to an image plane of an imaging element is long, in order to mount the imaging lens in a small camera such as cellular phones and smartphones, it is necessary to bend a light path with a prism or a mirror disposed between the imaging lens and the image plane. Functions and Sizes of cellular phones and smartphones are higher and smaller every year, and the level of downsizing required for the imaging lens is even higher than before. According to the lens configuration described in Patent Reference, it is difficult to attain satisfactory aberration while attaining downsizing of the imaging lens to meet those requirements.
Here, such problem is not a problem specific to the imaging lens to be mounted in cellular phones and smartphones. Rather, it is a common problem even for an imaging lens to be mounted in a relatively small camera such as digital still cameras, portable information terminals, security cameras, onboard cameras, and network cameras.
An object of the present invention is to provide an imaging lens that can satisfactorily correct aberrations. In addition, a second object of the present invention is to provide an imaging lens that can attain both downsizing of the imaging lens and satisfactorily corrected aberrations.
Further objects and advantages of the present invention will be apparent from the following description of the present invention.